MATH0028 Exam 2022
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All questions should be attempted. Marks obtained in all solutions will count.
Section A
1. (a) Solve the following recurrence: T (n) = 3T (n/4)+T (n/2)+n2 . You may assume that n is a power of 2, and that the initial value T (1) is positive. Express your answer as T (n) = Θ(f (n)) for some suitable function f .
(b) Which of the following are true? Justify your answers
❼ There is a function f with f (n) = O(n4 一 3n2 ) and f (n) = Ω(10n3 ).
❼ If g(n) = Θ ╱210 ^log2 n、, then g(n) = O(n2 ).
2. In the following, justify your answers
(a) Let G be a bipartite graph with 8 vertices in total, having 4 vertices in each
part. If G has no perfect matching, then what is the most edges that G can have?
(b) Consider five job applicants a1 , b1 , c1 , d1 , e1 applying to five firms a2 , b2 , c2 , d2 , e2 .
The preferences between the applicants and firms are given below.
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Preferences of applicants a1 , b1 , c1 , d1 , e1
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Preferences of firms a2 , b2 , c2 , d2 , e2
Find a stable matching in this preference profile. Is there a stable matching in which a1 gets paired with e2 ? What about a1 getting paired with a2 ?
(c) Let T be a tree in which every vertex has degree 1 or 2. Prove that T is a path.
3. Consider the decision problem SHORTCYCLE whose input is a directed graph G
on n vertices and whose output is YES whenever G has a cycle of length s n/2.
(a) Give a YES instance for the decision problem SHORTCYCLE. Give a NO in-
stance for the decision problem SHORTCYCLE.
(b) Give an algorithm for solving the decision problem SHORTCYCLE. Write down
a function so that the running time is O(f (n)).
(c) Is SHORTCYCLE in NP? Justify your answer.
4. (a) Consider the following graph with cost function as shown. Find a spanning tree whose cost is as large as possible in this graph. Justify your answer.
3
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3 4
3 3
1
k
4
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g
l
4 3
3
(b) Consider the following digraph with capacities as shown. For the edge ag the
capacity is some unknown positive real number x. For each x > 0, find the value of the maximum flow from a to k in the graph. Justify your answer.
(c) Give a Turing machine which tests for divisibility by 6. Show how your machine runs on the input 366.
2023-04-14